1. Field of the Invention
The invention relates to a method of controlling an adaptive filter, and more particularly, to a method of setting a transfer function of an adaptive filter.
2. Description of the Prior Art
With the development of science and technology, types of entertainment are getting more and more varied. For example, an accompaniment system can play the background music of songs allowing users to sing without a real band, providing a specialized form entertainment. For the requirement of the accompaniment system, background music without vocals are sold while the songs with vocals are also published. Users can then sing with the accompaniment system after listening the songs.
Because vocal ranges of people are different, the conventional accompaniment system has a key shifting function for adjusting the background music to harmonize it with different users. Please refer to FIG. 1, FIG. 2, and FIG. 3, which are schematic diagrams of the key shifting operation according to the prior art. If the background music includes a tone, which is superposed by three continuous signals with different frequencies in the time domain, the tone will separately correspond to frequencies f1, f2, and f3 in the frequency domain, as shown in FIG. 1.
If the first user is female, the tone of the first user will be generally higher. When the key of the background music is too low, the first user must lower her pitch to match the background music. Therefore, the first user can control the key shifting function to raise the key of the background music to match her tone. Please refer to FIG. 2, the original frequencies f1, f2, and f3 of the background music are adjusted with a coefficient “a” (a>1), and the raised frequencies a*f1, a*f2, and a*f3 are higher than the original ones. The first user can utilize the raised background music to sing without flatting the tone, or the first user can even use the key shifting function to raise the key of the background music to exceed her natural vocal range so that the first user can sharpen her tone to harmonize with the raised background music.
On the other hand, if the second user is male, the tone of the second user will be generally deep and suppressed. When the key of the background music is too high, the second user must sharpen the tone to match the background music. Therefore, the second user can control the key shifting function to lower the key of the background music to match his tone. Please refer to FIG. 3, the original frequencies f1, f2, and f3 of the background music are adjusted with a coefficient “b” (b<1), and the lowered frequencies b*f1, b*f2, and b*f3 are lower than the original ones. The second user can utilize the lowered background music to sing without sharpening the tone, or the second user can even use the key shifting function to modify the key of the background music to be lower than his tone so that the second user can flatten his tone to sing with the lowered background music.
There are many different methods to adjust the key of background music. For example, the period of signals of the background music is calculated and then is lengthened or shortened, and the signals with adjusted periods are superposed to complete the key shifting operation. When the user uses the key raising function, the adjusted background music will have more high frequency signals. The excessive frequencies will make the background music unpleasant, and the high frequency noise (produced by the signal source or the key shifting operation) will also disturb the adjusted background music. The conventional accompaniment system, therefore, always utilizes a filter to improve the final quality of the adjusted background music. Because the users can randomly control the key raising or lowering, the accompaniment uses an adaptive filter to adjust the filtering characteristic in accordance with different input data. The adaptive filter must expend much time to automatically adjust the frequency response, so the operation of the conventional adaptive filter is very heavy and complicated.